There is a continuing need in the petroleum refining and chemical industries for improved catalysts and process technology. One such process technology, hydroprocessing, has been subjected to increasing demands for improved heteroatom removal, aromatic saturation, and boiling point reduction. More active catalysts and improved reaction vessel designs are needed to meet this demand. Countercurrent reaction vessels have the potential of helping to meet these demands because they offer certain advantages over co-current flow reactors. Countercurrent hydroprocessing is well known, but of very limited commercial use. A countercurrent process is disclosed in U.S. Pat. No. 3,147,210 which teaches a two-stage process for the hydroprocessing-hydrogenation of high boiling aromatic hydrocarbons. The feedstock is first subjected to catalytic hydroprocessing, preferably in co-current flow with hydrogen. It is then subjected to hydrogenation over a sulfur-sensitive noble metal hydrogenation catalyst countercurrent to the flow of a hydrogen-rich gas. U.S. Pat. Nos. 3,767,562 and 3,775,291 disclose a similar process for producing jet fuels, except the jet fuel is first hydrodesulfurized prior to two-stage hydrogenation. U.S. Pat. No. 5,183,556 also discloses a two-stage concurrent-countercurrent process for hydrofining--hydrogenating aromatics in a diesel fuel stream.
An apparatus is disclosed in U.S. Pat. No. 5,449,501 that is designed for catalytic distillation. The distillation apparatus, which is a vessel, contains vapor passageways which provide a means for vapor communication between fractionation sections located above and below catalyst beds. Substantially all of the vapor in the vessel rises through the vapor passageways and the desired contacting between vapor and liquid occurs in the fractionation sections.
While the concept of countercurrent hydroprocessing has been known for some time, countercurrent flow reaction vessels are typically not used in the petroleum industry, primarily because conventional countercurrent reaction vessels are susceptible to catalyst bed flooding. That is, the relatively high velocity of the upflowing treat gas prevents the downward flow of the liquid. The liquid thus cannot pass through the catalyst bed. While flooding is undesirable, catalyst contacting by the reactant liquid improves as the bed approaches a flooded condition. However, operating close to the point of incipient flooding leaves the process vulnerable to fluctuations in pressure or temperature or in liquid or gas flow rates. This could result in a disturbance large enough to initiate flooding, and process unit shutdown in order to recover stable operation. Such disruptions are highly undesirable in a continuous commercial operation.
Therefore, there still exists a need for improved countercurrent reaction vessel designs which are not as readily susceptible to flooding, or which can more easily recover without shutdown should flooding occur.